Electrode including nonmetallic substrate



United States Patent 1 Claim. (Cl. 136-120) This is a division of application Ser. No. 162,221, filed Dec. 26, 1961', now Patent No. 3,235,473.

This invention relates to an improved electrode and method for the manufacture thereof. In one aspect the invention relates to an improved gas diffusion electrode of low resistivity. In another aspect this invention relates to a method of making a porous electrode having a continuous layer of conductive material on an inert substrate.

Commercialization of fuel cells has been handicapped by the difiiculty experienced in providing electrodes having a low resistivity and which are inexpensive. Low resistance is especially important with respect to fuel cell electrodes since such cells usually generate current of low voltage. Conventional gas electrodes usually comprise carbon, metal powders or specially prepared porous metals. A disadvantage in using carbon electrodes is that heavier electrodes are required in order to obtain high current capacities. A drawback to the use of loose powdered metals in a pre-formed container is that it is difficult to control the voids between the particles so that the amount of gas which passes through the electrode Without being absorbed is minimized. In the case of powder electrodes, it has been reported that a resinous thermoplastic may be used to act as a binder and to facilitate subsequent molding of theelectrode. Although this latter type of electrode employs less electroconductive material than electrodes consisting of solid metal or metal powders, it has generally been believed that it isimportant to minimize the total quantity ofbinder in order to obtain satisfactory electrical conductivity. In additionto the fact that such electrodes are not too much less in cost than electrodes consisting of metal powder or solid metal, continuous contact of the particles of metal with each other by mere mechanical mixing is not achieved since the random distribution ofresinous particles necessarily leads to separation of the metal particles by those of non-conductive binder. v

In practice, gas diffusion electrodes whether fabricated of solid metal or powder are usually mounted on, or containedin, a perforated steel grid, screen or plate. Due to the fact that such supports and the conductive metal are held together by a weak mechanical metal-to-metal bond, the bond becomes weakened during subsequent fabrication and use thereby affecting the performance. The same type of difliculty is usually encountered with electrodes made by pasting metal salts onto a perforated metal support and converting the salt to elemental metal either by thermal or electrochemical techniques. Aside from overcoming such drawbacks, it also is desired to provide a process for producing electrodes which are more amenable to being formed into a variety of shapes.

It is an object of this invention to provide an electrode in which the total amount of electroconductive metal is reduced without sacrificing conductivity.

Another object is to provide an improved method for Another object of this invention is to provide an electrode particularly useful in fuel cells in which a gas or liquid is used as fuel which electrodes have a minimum content of electrically conductive material, good physical strength and which are self-supporting.

A further object is to provide an improved gas diffusion electrode particularly useful in fuel cells in which an oxidizing gas such as oxygen or chlorine is used as a reactant.

A further object is to provide a gas dilfusion electrode comprising silver and a diluent which electrode has conductivity properties comparable to that of a solid silver electrode.

A further object is to provide a process for the manufacture of electrodes having the above characteristics which process allows for the production of electrodes of a variety of shapes and forms, and is readily adapted to continuous operation.

A still further object is to provide an improved and economical method for producing electrodes having catalytic agents incorporated therein.

Various other objects and advantages of this invention will become apparent to those skilled in the art from the accompanying description and disclosure.

In accordance with the teachings of the invention, an electrode is provided, comprising an inert substrate having deposited on its surface a continuous film of electrically conductive material. By this invention, there also is provided a method of making an electrode particularly useful as a gas or liquid dilfusion electrode, which method allows for the production of electrodes having a minimum content of electroconductive material without sacrificing desirable conductivity properties. By the process of this invention an inert solid such as polyethylene, for example, is contacted with a compound of an electroconductive metal and said compound is converted to elemental metal on the surface of the substrate as an adherent and continuous layer.

The inert substrate functions as a carrier and diluent for the electroconductive material and may be any solid commonly regarded as a non-conductor or electrical insulator, or as a poor conductor of electricity. Depending upon the particular end use of the completed electrode, the inert carrier may be porous or non-porous. When it is desired to produce gas or liquid diffusion electrodes, it is preferred to employ the inert substrate in a porous form. Suitable inert carriers are: polymeric materials including thermoplastics and synthetic and natural elastomers, typical examples of which are polyethylene, polypropylene, polyvinyl chloride, polystyrene, polyvinyl acetate, polybutadiene, polyisoprene, polychloroprene, poly trifluorochloroethylene, polytetrafiuoroethylene, the thermoplastic and elastomeric polymers produced by copolymerization of trifiuorochloroethylene with vinylidene fluoride or tetrafiuoroethylene, condensation products of formaldehyde such as formaldehyde-phenol (Bakelite) and formaldehyde-urea plastics and casein; ion exchange materials of both the anionic and cationic type such as amberlites, halfilm, etc.; low conductors such as ceramics, metal oxide refractory materials, typical examples of which are titania, alumina, zirconia, silica, thoria, magnesia, zeolites, chromia, magnetite, porous graphite and asbestos; woven textiles such as woven graphite, woven asbestos, woven glass and woven inorganic fibers, deposited mats; parchments; porous glass, asbestos papers, Teflon coated fiber glass (woven) and other such materials generally regarded as being relatively inert to United States Patent 1 Claim. (Cl. 136-120) This is a division of application Ser. No. 162,221, filed Dec. 26, 1961', now Patent No. 3,235,473.

This invention relates to an improved electrode and method for the manufacture thereof. In one aspect the invention relates to an improved gas diffusion electrode of low resistivity. In another aspect this invention relates to a method of making a porous electrode having a continuous layer of conductive material on an inert substrate.

Commercialization of fuel cells has been handicapped by the difiiculty experienced in providing electrodes having a low resistivity and which are inexpensive. Low resistance is especially important with respect to fuel cell electrodes since such cells usually generate current of low voltage. Conventional gas electrodes usually comprise carbon, metal powders or specially prepared porous metals. A disadvantage in using carbon electrodes is that heavier electrodes are required in order to obtain high current capacities. A drawback to the use of loose powdered metals in a pre-formed container is that it is difficult to control the voids between the particles so that the amount of gas which passes through the electrode Without being absorbed is minimized. In the case of powder electrodes, it has been reported that a resinous thermoplastic may be used to act as a binder and to facilitate subsequent molding of theelectrode. Although this latter type of electrode employs less electroconductive material than electrodes consisting of solid metal or metal powders, it has generally been believed that it isimportant to minimize the total quantity ofbinder in order to obtain satisfactory electrical conductivity. In additionto the fact that such electrodes are not too much less in cost than electrodes consisting of metal powder or solid metal, continuous contact of the particles of metal with each other by mere mechanical mixing is not achieved since the random distribution ofresinous particles necessarily leads to separation of the metal particles by those of non-conductive binder. v

In practice, gas diffusion electrodes whether fabricated of solid metal or powder are usually mounted on, or containedin, a perforated steel grid, screen or plate. Due to the fact that such supports and the conductive metal are held together by a weak mechanical metal-to-metal bond, the bond becomes weakened during subsequent fabrication and use thereby affecting the performance. The same type of difliculty is usually encountered with electrodes made by pasting metal salts onto a perforated metal support and converting the salt to elemental metal either by thermal or electrochemical techniques. Aside from overcoming such drawbacks, it also is desired to provide a process for producing electrodes which are more amenable to being formed into a variety of shapes.

It is an object of this invention to provide an electrode in which the total amount of electroconductive metal is reduced without sacrificing conductivity.

Another object is to provide an improved method for Another object of this invention is to provide an electrode particularly useful in fuel cells in which a gas or liquid is used as fuel which electrodes have a minimum content of electrically conductive material, good physical strength and which are self-supporting.

A further object is to provide an improved gas diffusion electrode particularly useful in fuel cells in which an oxidizing gas such as oxygen or chlorine is used as a reactant.

A further object is to provide a gas dilfusion electrode comprising silver and a diluent which electrode has conductivity properties comparable to that of a solid silver electrode.

A further object is to provide a process for the manufacture of electrodes having the above characteristics which process allows for the production of electrodes of a variety of shapes and forms, and is readily adapted to continuous operation.

A still further object is to provide an improved and economical method for producing electrodes having catalytic agents incorporated therein.

Various other objects and advantages of this invention will become apparent to those skilled in the art from the accompanying description and disclosure.

In accordance with the teachings of the invention, an electrode is provided, comprising an inert substrate having deposited on its surface a continuous film of electrically conductive material. By this invention, there also is provided a method of making an electrode particularly useful as a gas or liquid dilfusion electrode, which method allows for the production of electrodes having a minimum content of electroconductive material without sacrificing desirable conductivity properties. By the process of this invention an inert solid such as polyethylene, for example, is contacted with a compound of an electroconductive metal and said compound is converted to elemental metal on the surface of the substrate as an adherent and continuous layer.

The inert substrate functions as a carrier and diluent for the electroconductive material and may be any solid commonly regarded as a non-conductor or electrical insulator, or as a poor conductor of electricity. Depending upon the particular end use of the completed electrode, the inert carrier may be porous or non-porous. When it is desired to produce gas or liquid diffusion electrodes, it is preferred to employ the inert substrate in a porous form. Suitable inert carriers are: polymeric materials including thermoplastics and synthetic and natural elastomers, typical examples of which are polyethylene, polypropylene, polyvinyl chloride, polystyrene, polyvinyl acetate, polybutadiene, polyisoprene, polychloroprene, poly trifluorochloroethylene, polytetrafiuoroethylene, the thermoplastic and elastomeric polymers produced by copolymerization of trifiuorochloroethylene with vinylidene fluoride or tetrafiuoroethylene, condensation products of formaldehyde such as formaldehyde-phenol (Bakelite) and formaldehyde-urea plastics and casein; ion exchange materials of both the anionic and cationic type such as amberlites, halfilm, etc.; low conductors such as ceramics, metal oxide refractory materials, typical examples of which are titania, alumina, zirconia, silica, thoria, magnesia, zeolites, chromia, magnetite, porous graphite and asbestos; woven textiles such as woven graphite, woven asbestos, woven glass and woven inorganic fibers, deposited mats; parchments; porous glass, asbestos papers, Teflon coated fiber glass (woven) and other such materials generally regarded as being relatively inert to solution so that the solution is drawn through the pores of the material.

In accordance with another embodiment of the process of this invention, a continuous layer of the conductor is deposited on the surface and in the pores of the substrate by the method which comprises contacting vapors of the conductor with the substrate, under vacuum. This technique comprises heating the metal to be deposited in a system evacuated to to 10 mm. mercury pressure and at a temperature at which the vapor pressure of the metal becomes appreciable. The vapors are allowed to deposit on the surface of the substrate which is maintained at room temperature. The temperature at which some representative metals are vaporized are: aluminum (1000 (3.), copper (1270 0.), gold (1400 C.), silver (1050 C.) and zinc (340 C.). For example, in accordance with this method, silver is deposited on the surface and in the pores of a porous plastic substrate by placing the substrate in a chamber evacuated to about 10- to about 10- mm. mercury and equipped with means for heating silver to about 1050 C. Heating of the silver is accomplished by placing it in or on a tungsten filament which is heated by passing an electric current therethrough. Other methods of heating the silver include placing it in a crucible which is heated, or by passing a current through a silver wire.

Other methods particularly useful for depositing a continuous film of conductor on the surface of and in the pores of a porous substrate, include sputtering and electrophoresis. The sputtering technique involves utilization of the metal to be deposited as a cathode in a system evacuated to approximately 0.3 to about 0.5 micron of mercury pressure and subjecting it to a high voltage of the order of between about 1000 and about 10,000 volts or higher. The metal is thereby stripped from the face of the cathode, travels to the substrate and deposits thereon as a continuous film. Deposition by electrophoresis is accomplished by placing the substrate between two inert electrodes in a vessel containing a colloidal suspension of the electroconductive material in a vehicle of low conductivity such as isopropanol or butanol, and impressing a large potential difference of the order of between about 200 and about 1000 volts direct current or higher on the electrode system.

Other methods of deposition include impregnation of the substrate with a solution, slurry or vapor comprising the compound to be deposited and reducing the metal ions to free metal on the surface of the substrate at elevated temperatures or by chemical reaction.

It is to be understood that although the above methods are described with particular reference to the manufacture of gas or liquid diffusion electrodes comprising the conductor evenly distributed throughout an inert porous material, the methods are also advantageously employed in depositing the conductor on the surface of a nonporous material.

By the process of this invention electrodes are provided having good conductivity and minimum metal contents. Notwithstanding the fact that the electroconductive material is diluted by an inert material, the process of this invention allows for the formation of electrodes having conductivity equivalent to that of a solid conductor without a material increase in the resistivity. It is known that the various metals differ in their ability to conduct electricity and at any given current there is a minimum mass required to conduct the electricity with only a negligible rise in temperature. The minimum mass requirements are defined by the following equation:

=K l l wherein:

M =m'ass of the metal in grams l=current carried by an electrode of cross-sectional area,

A (heightXwidth) 6 l=length of the electrode K=D/C wherein:

D=density of the conductor C=maximum current capacity of the conductor per unit area with negligible rise in temperature of the conductor The above equation was used to calculate the minimum mass of conduct-or required to pass a current of 10 amperes across the electrode, basing the maximum current capacity of the various materials on the maximum current capacity of copper, taken as 6001000 amperes per square inch, 1 being 9.5 centimeters. The following Table I is a tabulation of the density of various conductors measured at 20 C.; the maximum current capacity per square centimeter (C) using copper as the standard; and the minimum mass (M) required to pass a current of 10 amperes calculated using the above equation.

TAB LE 1 Maximum current capacity Conductor meter (amperes/ (M) (grams) (basis 1,000 amps/in?) 2. 7 101 2. 54 8. 5 0. 078 4, 000 2. 5 0. 33 720 7. 1 101 6. 7 8. 0 27 31 8. 9 155 5. 5 19 110 16 7. 9 26 29 8. 9 38 22. 3 12 24 38 21 24 83 11 164 6. 4 17 20 I 4. 5 88 4. 9 Zircomum 6. 4 1. 6 380 Inspection of the minimum mass requirements tabulated in the above Table I shows that of the various matreials listed, aluminum, chromium, copper, silver and titanium are comparable in terms of mass requirements which are very low as compared with the other conductors. The process of this invention makes it possible to take advantage of the fact that certain metals such as silver and titanium need be used in only very small amounts to obtain maximum conductivity. Thus, since the substrate functions as both a support and diluent for the conductor and, since the method of this invention leads to the distribution of conductor as a continuous film on and within the substrate, self-supporting electrodes are provided having conductivity properties comparable to those of a solid conductor at a minimum mass of metal. On the other hand, prior methods such as those involving the production of solid metal conductors cannot take advantage of minimum mass requirements in view of the fact that, if minimum amounts of metal such as silver or titanium are used, the electrodes would necessarily be very thin and not self-supporting thereby requiring bonding to a rigid support by special welding or mechanical bonding techniques.

In accordance with this invention, the weight ratio of inert substrate to electroconductive metal varies over a relatively wide range such as between about 10:1 and about 1: 1, the particular ratio depending largely upon the conductivity of the metal deposited. For example, in the case of the more highly conductive metals such as silver and titanium, the weight ratio of substrate to metal may be as high as between about 6:1 and about 3:1, although it is to be understood that the electrode may contain heavier deposits of metal without departing from the scope of this invention.

It is to be understood that electrodes containings catalysts for fuel cell reactions or for catalyzing the electrochemical synthesis of organic compounds, may be produced in accordance with this invention. Thus, catalysts such as platinum, palladium, gold, silver, nickel, cobalt, iron, manganese, rare earth metals, etc., and their various oxides and salts and any combinations thereof are advantageously incorporated in the electrode either simultaneously with the deposition of the conductor or in a subsequent step. The catalysts are deposited on the electrode surface and inner area by the various techniques described above including the electroless and electroplating techniques, vapor deposition, sputtering, electrophoretic deposition, impregnation with catalytic precursors and subsequent decomposition to the catalytic agent, as well as by anodization of the electrode surface either with or without pretreatment with reducible salts.

In addition to the fact that the process of this invention leads to the production of electrodes of good conductivity properties at a minimum mass of metal, the porosity, surface area, shape and structure of the electrode are readily controlled. Desired porosity and surface area of the gas or liquid diffusion electrodes of this invention are obtained by controlling the surface of the substrate which is readily accomplished. For example, in accordance with the preferred embodiment of this invention, a flexible plastic or elastomeric substrate is used, the surface areas and porosity of which are controlled to preferred sizes or dimensions by methods of fabrication, by surface treatment such as heat treatment or compression, or by treating the substrate with a liquid capable of penetrating the substrate to cause pore enlargement. For example, treatment of polyethylene with a hydrocarbon solvent such as tetralin or decalin causes enlargement of the pore size. In addition, diffusion electrodes of increased surface area are provided by depositing the electroconductive metal on a substrate, the surface of which has been roughened or machined to provide a velour or sandpaperlike surface, or fibrillated surface. Another metal of producing gas or liquid diffusion electrodes of high surface area and having a maximum of reaction sites comprises first depositing a thin continuous layer of conductor within the pores of a porous substrate as described herein, followed by the deposition of a heavier layer of fine crystallites of conductor on the outer surface by electroplating at a controlled rate.

Thus by the process of this invention gas or liquid diffusion electrodes are provided in which the electroconductive metal is deposited as an adherent film having continuity within the substrate as well as on the outer surface thereof. Such continuity of contact is not achieved by mere mechanical mixing of an inert material such as plastics in the form of discrete particles with metal powder since it is not possible thereby to obtain continuous contact between the particles of metal.

In addition to the above advantages, the process of this invention is readily adapted to the manufacture of electrodes of any desired shape, merely by preforming the substrate into the shape desired for the final electrode, thereby eliminating the necessity for machining or mechanical processing of the electrodes after deposition of the metal. From this standpoint, therefore, the inert substrate is a readily moldable material such as one of the aforesaid plastic or elastomeric materials, the molding of such substrates being well known to those skilled in the art. For example, when the electrodes of this invention are to be used in gas fuel cells, polyethylene is molded into the form of a hollow porous body of any desired cross-sectional shape such as circular or rectangular, and then subjected to the plating methods described above to deposit the conductor on its surface in any desired thickness, and within the body of the porous substrate to any desired depth.

A particularly preferred form of self-supporting fluid diffusion electrode of either the gas or liquid type cornprises a series of contiguous tubes the surface of which has been metallized by the method of this invention. Such electrode assemblies are prepared by molding a flexible porous substrate such as one of the above-mentioned plastics or elastomers into corrugated sheets having a substantially sinuous cross section, aligning the sheets by placing the elevations or peaks of one sheet in opposing relationship to the depressions or valleys of the other sheet such that they are brought into contact and enclose cylindrical voids, and heat sealing the sheets along the vertical plane of contact. A continuous layer of electroconductive metal is then deposited on the outer surface, which may be a velour or fibrillated surface, and within the pores of the substrate as described herein, the particularly preferred method being electroless flash plating followed by electroplating including prior sensitization of the substrate. The series of contiguous tubes can also be formed by continuous extrusion of a plastic or admixtures which subsequently can be rendered porous.

As described above, catalytic agents may be incorporated within the surface of the metallized electrode itself. Also included within the scope of this invention, is the production of catalytically active electrode assemblies comprising a layer of porous substrate having a catalyst incorporated therein by the deposition techniques described herein, sandwiched between two electrodes which may or may not be electrodes produced by the process of this invention. The preferred form of this type of assembly is prepared, for example, by positioning said catalyst layer between two electrodes (anode and cathode) one or both of which have been metallized by the process of this invention, the electrodes and catalytic layer being held or bonded together by compression or heat sealing techniques. This form of catalytic electrode may also include a current distributor such as a metal screen or perforated plate positioned between the catalyst-containing layer and the electrodes.

The following examples are offered as a better understanding of the present invention and are not to be construed as unnecessarily limiting thereto.

In each experiment, a strip of porous polyethylene (4 inches square and A3 inch thick) was mounted horizontally in a plastic housing which was connected to a vacuum reservoir having a three-way valve attached to a vacuum line and an air line. The plastic housing was then positioned in a glass dish to which the various treating solutions were added. In each case, the glass dish was filled with a sufficient amount of the particular treating solution so as to immerse the polyethylene mounted in the plastic casing. Circulation of the treating solution through the porous polyethylene was achieved by first reducing the pressure in the vacuum reservoir to draw the solution upwardly through the porous polyethylene into the upper portion of the plastic housing above the polyethylene, followed by increasing the pressure with air to force the solution downwardly through the polyethylene back into the glass dish. In this manner, the polyethylene was cleaned by filling the glass dish with standard sulfuric acid-sodium dichromate cleaning solution at about 50 C. by circulating it through the porous polyethylene for about 3 minutes. The polyethylene was then washed thoroughly by filling the dish with distilled water and circulating the water through the polyethylene for about 3 minutes. The glass dish was then filled with a dilute aqueous solution of stannous chloride (0.01 molar) and the polyethylene treated therewith by alternately decreasing and increasing the pressure over the mounted strip so as to force the solution therethrough. After about 3 minutes of circulating the stannous chloride solution through the polyethylene, the polyethylene was air dried for about 5 minutes. The substrate was then treated with a dilute aqueous solution of palladous chloride (0.01 molar) using the above-described circulation technique, for about 3 minutes, the stannous ion on the surface of the polyethylene reducing the palladous ion to elemental palladium. The sensitized polyethylene was then dried in air for about 10 minutes and then treated so as to deposit a thin layer of copper on its surface. Flash plating of duced in accordance with this invention. Thus, catalysts such as platinum, palladium, gold, silver, nickel, cobalt, iron, manganese, rare earth metals, etc., and their various oxides and salts and any combinations thereof are advantageously incorporated in the electrode either simultaneously with the deposition of the conductor or in a subsequent step. The catalysts are deposited on the electrode surface and inner area by the various techniques described above including the electroless and electroplating techniques, vapor deposition, sputtering, electrophoretic deposition, impregnation with catalytic precursors and subsequent decomposition to the catalytic agent, as well as by anodization of the electrode surface either with or without pretreatment with reducible salts.

In addition to the fact that the process of this invention leads to the production of electrodes of good conductivity properties at a minimum mass of metal, the porosity, surface area, shape and structure of the electrode are readily controlled. Desired porosity and surface area of the gas or liquid diffusion electrodes of this invention are obtained by controlling the surface of the substrate which is readily accomplished. For example, in accordance with the preferred embodiment of this invention, a flexible plastic or elastomeric substrate is used, the surface areas and porosity of which are controlled to preferred sizes or dimensions by methods of fabrication, by surface treatment such as heat treatment or compression, or by treating the substrate with a liquid capable of penetrating the substrate to cause pore enlargement. For example, treatment of polyethylene with a hydrocarbon solvent such as tetralin or decalin causes enlargement of the pore size. In addition, diffusion electrodes of increased surface area are provided by depositing the electroconductive metal on a substrate, the surface of which has been roughened or machined to provide a velour or sandpaperlike surface, or fibrillated surface. Another metal of producing gas or liquid diffusion electrodes of high surface area and having a maximum of reaction sites comprises first depositing a thin continuous layer of conductor within the pores of a porous substrate as described herein, followed by the deposition of a heavier layer of fine crystallites of conductor on the outer surface by electroplating at a controlled rate.

Thus by the process of this invention gas or liquid diffusion electrodes are provided in which the electroconductive metal is deposited as an adherent film having continuity within the substrate as well as on the outer surface thereof. Such continuity of contact is not achieved by mere mechanical mixing of an inert material such as plastics in the form of discrete particles with metal powder since it is not possible thereby to obtain continuous contact between the particles of metal.

In addition to the above advantages, the process of this invention is readily adapted to the manufacture of electrodes of any desired shape, merely by preforming the substrate into the shape desired for the final electrode, thereby eliminating the necessity for machining or mechanical processing of the electrodes after deposition of the metal. From this standpoint, therefore, the inert substrate is a readily moldable material such as one of the aforesaid plastic or elastomeric materials, the molding of such substrates being well known to those skilled in the art. For example, when the electrodes of this invention are to be used in gas fuel cells, polyethylene is molded into the form of a hollow porous body of any desired cross-sectional shape such as circular or rectangular, and then subjected to the plating methods described above to deposit the conductor on its surface in any desired thickness, and within the body of the porous substrate to any desired depth.

A particularly preferred form of self-supporting fluid diffusion electrode of either the gas or liquid type cornprises a series of contiguous tubes the surface of which has been metallized by the method of this invention. Such electrode assemblies are prepared by molding a flexible porous substrate such as one of the above-mentioned plastics or elastomers into corrugated sheets having a substantially sinuous cross section, aligning the sheets by placing the elevations or peaks of one sheet in opposing relationship to the depressions or valleys of the other sheet such that they are brought into contact and enclose cylindrical voids, and heat sealing the sheets along the vertical plane of contact. A continuous layer of electroconductive metal is then deposited on the outer surface, which may be a velour or fibrillated surface, and within the pores of the substrate as described herein, the particularly preferred method being electroless flash plating followed by electroplating including prior sensitization of the substrate. The series of contiguous tubes can also be formed by continuous extrusion of a plastic or admixtures which subsequently can be rendered porous.

As described above, catalytic agents may be incorporated within the surface of the metallized electrode itself. Also included within the scope of this invention, is the production of catalytically active electrode assemblies comprising a layer of porous substrate having a catalyst incorporated therein by the deposition techniques described herein, sandwiched between two electrodes which may or may not be electrodes produced by the process of this invention. The preferred form of this type of assembly is prepared, for example, by positioning said catalyst layer between two electrodes (anode and cathode) one or both of which have been metallized by the process of this invention, the electrodes and catalytic layer being held or bonded together by compression or heat sealing techniques. This form of catalytic electrode may also include a current distributor such as a metal screen or perforated plate positioned between the catalyst-containing layer and the electrodes.

The following examples are offered as a better understanding of the present invention and are not to be construed as unnecessarily limiting thereto.

In each experiment, a strip of porous polyethylene (4 inches square and A3 inch thick) was mounted horizontally in a plastic housing which was connected to a vacuum reservoir having a three-way valve attached to a vacuum line and an air line. The plastic housing was then positioned in a glass dish to which the various treating solutions were added. In each case, the glass dish was filled with a sufficient amount of the particular treating solution so as to immerse the polyethylene mounted in the plastic casing. Circulation of the treating solution through the porous polyethylene was achieved by first reducing the pressure in the vacuum reservoir to draw the solution upwardly through the porous polyethylene into the upper portion of the plastic housing above the polyethylene, followed by increasing the pressure with air to force the solution downwardly through the polyethylene back into the glass dish. In this manner, the polyethylene was cleaned by filling the glass dish with standard sulfuric acid-sodium dichromate cleaning solution at about 50 C. by circulating it through the porous polyethylene for about 3 minutes. The polyethylene was then washed thoroughly by filling the dish with distilled water and circulating the water through the polyethylene for about 3 minutes. The glass dish was then filled with a dilute aqueous solution of stannous chloride (0.01 molar) and the polyethylene treated therewith by alternately decreasing and increasing the pressure over the mounted strip so as to force the solution therethrough. After about 3 minutes of circulating the stannous chloride solution through the polyethylene, the polyethylene was air dried for about 5 minutes. The substrate was then treated with a dilute aqueous solution of palladous chloride (0.01 molar) using the above-described circulation technique, for about 3 minutes, the stannous ion on the surface of the polyethylene reducing the palladous ion to elemental palladium. The sensitized polyethylene was then dried in air for about 10 minutes and then treated so as to deposit a thin layer of copper on its surface. Flash plating of 

1. A CATALYTICALLY ACTIVE ELECTRODE ASSEMBLY WHICH COMPRISES A LAYER OF A POROUS INERT MATERIAL HAVING DEPOSITED WITHIN THE PORES THEREOF A CATALYTIC AGENT, SAID LAYER BEING POSITIONED BETWEEN TWO POROUS ELECTRODES COMPOSED OF A POROUS NON-METALLIC SUBSTRATE HAVING DEPOSITED THEREON AND WITHIN THE PORES THEREOF AN ADHERENT AND CONTINUOUS LAYER OF ELECTROCONDUCTIVE METAL AND HAVING A CURRENT DISTRIBUTING MEANS BETWEEN SAID LAYER AND SAID ELECTRODES. 